In recent years, a power module has been widely used in a power converter using an insulated gate bipolar transistor (IGBT) as a main device. The power module incorporates one or a plurality of power semiconductor chips to form a portion or all of a conversion connection.
Generally, the power module has a structure in which the power semiconductor chips are electrically insulated from a metal substrate or a cooling surface in such a manner that the power semiconductor chips are mounted on a ceramic insulation substrate and that the power semiconductor chips and the ceramic insulation substrate are accommodated in a case and the ceramic insulation substrate is solder-joined to a metal base plate at the same time. In order to insulate and protect a creepage of the ceramic insulation substrate and the power semiconductor chips on the substrate, a low-elastic-modulus silicone gel is filled in the case.
In an existing power module, an upper limit of an operation temperature is 150° C. However, an automobile power module or a server power module may be often used at a temperature condition exceeding 150° C. For example, Tj (temperature of jointed portion between chips) can reach instantaneously 175° C., and thus the power module is required to normally operate at such a high temperature.
In order to ensure an operation at 175° C. for 100,000 hours by an UL 1557 standard, one of lifetime acceleration test conditions listed in Table 3.1 of such a standard is selected and it is necessary to maintain an insulation breakdown withstand voltage of a product specification when being tested at the selected condition. When a heating test is performed at the lifetime acceleration test condition corresponding to the operation at 175° C. for 100,000 hours, for example, at a condition of 200° C. and 6,000 hours from Table 3.1 of UL 1557, in the existing IGBT power module, cracks occur in the silicone gel filled in the case to nearly reach the creepage of the insulation substrate and an insulation breakdown voltage is not satisfied with the standard.
In a general silicone oil or rubber, it is considered that heat resistance is improved by a method of adding filler such as carbon black, iron oxide, or titanium oxide. However, as a silicone gel material for the IGBT power module in which low viscosity and insulation properties are required, the method by the addition of the carbon black, iron oxide, titanium oxide or the like is not a way to be accepted easily since disadvantages such as reduction in the insulation properties, sedimentation of filler, or reduction in workability due to an increase of viscosity occur. Further, even in the silicone gel to which the filler for improving the heat resistance is added, the silicone gel which can achieve a lifetime acceleration test condition at 175° C. has not been found.
With respect to a silicone composition, there is a heat-resistant silicone composition prepared by dissolving or dispersing one or two or more fullerenes in silicone oil (JP 2005-206761 A). However, since the silicon composition disclosed in JP 2005-206761 A is prepared by dissolving or dispersing the fullerenes having conductivity, there is a risk of deterioration of the electrical insulation property.